GASTEROPODA, 



235 



stance, which, while it prevents it from crawling, 

 gives it the faculty of swimming at the surface of 

 the water. " When the sea is calm, these animals 

 may be seen collected often in large bands, swim- 

 ming over the surface, by means of this floating 

 apparatus of bubbles, aided by a fin on each side 

 of the body. During this action, the head is very 

 prominent, and the foot is so extended, that the 

 float, or line of vesicles, forms an angle with the 

 middle of the shell. When the sea is rough, the 

 animal absorbs the air from its vesicles, changes the 

 direction of its foot, contracts its body, and lets 

 itself sink. It does the same when in danger from 

 any enemy, and further, like the Cuttle-fish and 

 some others, colours the water by the emission of 

 a blue fluid, which serves to conceal it. They are 

 vividly phosphoric in the night. Birds carry them 

 off with great dexterity. 



" If their floating apparatus is mutilated, the 

 foot can reproduce it. The violet coloured shell 

 of this little animal is remarkably thin, which 

 facilitates its excursions on the surface. It is singu- 

 lar, that under its fragile float, a little line of pearly 

 fibres may be perceived, to which are attached its 

 eggs ; in some species they are contained in little 

 membranous bags, or sacs. It is thought that the 

 young animals, when liberated from these bags, or 

 chambers, ascend their mother's float, and so are 

 transported to the surface. Fishes are enabled to 

 rise to the surface of the water, by means of their 

 air-bladders, and some Radiaries by a vesicle which 



